In my opinion it could only be done in the user library of faces. When scrolling through the faces you're using, have saved, bought or whatever, have a star or other indicator on it letting them know there's an update for the face. Or possibly a tag below saying 'updated'.
Notifications, or anything else could be overwhelming and become meaningless.
I buy and sell items at Envato's website (After Effects projects on Videohive and Code Canyon scripts) and the amount of update notices quickly caused me to filter them because they were constant.